1. Field of the Invention
The present invention relates to a copolymer suitable for a stretch film and a wrap film in particular, a process for producing said copolymer, and a molded article of said copolymer.
2. Description of the Related Arts
In general, an elastic recoverability, transparency and mechanical strength found in a polyvinyl chloride are required for a copolymer to be used as a stretch film, a shrink film or a wrap film. However, the polyvinyl chloride comes in question concerning environmental pollution because of the possibility of generation of harmful substances in incineration. Various kind of polymers are presently considered as a replacement for the polyvinyl chloride, but it is a status quo that none of them being satisfactory from the viewpoints of viscoelastic properties and transparency has been obtained.
Since copolymerization of an .alpha.-olefin represented by propylene with an alkenyl aromatic hydrocarbon represented by styrene was carried out using a Solvay type titanium trichloride catalyst, studies using various solid catalysts have been carried out. For example, polymerization processes which use a solid catalyst using titanium tetrachloride and an electron donor are disclosed in Japanese Patent Publication (Kokai) Nos. 60-26011 and 4-318006. However, in both of those processes, the polymerization activity is low and a homopolymer is formed as a by-product. Further, the obtained copolymer has a low amount of copolymerized styrene and possesses some crystallinity, and is not adequate from the viewpoints of elastic recoverability, flexibility and transparency.
Japanese Patent Publication (Kokai) No. 8-269134 discloses a process for producing a copolymer of propylene with styrene which has syndiotactic propylene sequences using a so-called homogeneous Ziegler-Natta catalyst which uses a specific transition metal compound and an organoaluminum compound. However, the copolymer obtained by the process, has also a low styrene content and a crystallinity, and is insufficient from the viewpoints of elastic recoverability, flexibility and transparency.